1. Field of the Invention
The present invention relates to a optical recording medium capable of recording, reproducing, and rewriting information by irradiating laser to cause optical alternation on a material that constitutes a recording layer (hereinafter sometimes referring to as “phase change optical recording medium”, “optical recording medium”, “optical information recording medium” or “information recording medium”), a sputtering target for producing the optical recording medium, a process for using the recording medium, and an optical recording apparatus.
2. Description of Related Art
Recently, optical recording media are demanded to address recording with higher velocity. In case of disc optical recording media in particular, the higher velocity has been practically achieved on the grounds that raising the rotating velocity can increase the velocity of recording and reproducing. The recording media, on which the recording can be performed solely by modulation of optical intensity among others, may be commercially available in lower price owing to the simple recording mechanism. Also the reproducing mechanism based on the modulation of optical intensity affords effective interchangeability with reproducing-only apparatuses and makes possible the broad market; therefore, such optical recording media have been demanded the higher density and higher velocity recording owing to the recent extended capability of electric information.
Nowadays, the optical recording media, which employ phase change materials, are popular since plural times of rewriting are possible among others. The optical recording media, which employ phase change materials in the recording layer, perform through forming rapidly cooled and slowly cooled conditions by means of intensity modulation of the irradiated optical beams. In a rapidly cooled condition after melting, the material of recording layer turns into an amorphous state, whereas in a slowly cooled condition after melting, the material of recording layer turns into a crystal state. The optical properties are different between the amorphous and crystal conditions; thereby information may be recorded and reproduced.
The mechanism is that heating a recording layer through irradiating laser light on a thin film of the recording layer on a substrate, causing phase changes of the recording layer between the amorphous and crystal conditions, and repeatedly recording information by changing the disc reflectance. In many cases, the un-recorded condition corresponds to the crystal phase; information is recorded on the recording layer by forming marks of amorphous phase with lower reflectance and spaces of crystal phase with higher reflectance.
As well-known, recording light is arranged into pulses of which the intensity is modulated three levels, and the recording is performed through irradiating the recording light onto an optical recording medium. FIG. 6 exemplifies a pattern of emission waveform or recording strategy for repeatedly recording data of marks and spaces, which is employed in DVD+RW etc. The marks of amorphous state are formed by alternately irradiating peak power light (Pp=Pw) and bias power light (Pb) in pulse fashion; the spaces of crystal state are formed by continuously irradiating erase power light (Pe) that has an intermediate level of Pp and Pb.
Irradiating pulse arrays of peak power light and bias power light leads to formation of amorphous marks through the repeated melting and rapid cooling of the recording layer. Irradiating the erase power light leads to formation of spaces since the recording layer is melted then slowly cooled or is annealed in the solid state.
The formation of amorphous marks requires a melted state of the recording layer. In the higher velocity recording, higher power is required since the period of irradiating the peak power light is shortened. However, the power of laser diode (LD) is limited in the output power, resulting in inferior amorphous marks due to the insufficient output power. Accordingly, lower melting points are demanded for the materials of recording layer for higher velocity recording.
Various materials have been proposed heretofore with respect to the recording layer. The materials of recording layer in practice for CD-RW, DVD+RW, DVD-RW, DVD-RAM etc. can be broadly divided into Ag—In—Sb—Te family and Ge—Sb—Te family. The Ag—In—Sb—Te family is constituted by adding Ag and In into delta-phase solid solution of Sb—Te binary system that contains 63 to 83 atomic percent of Sb. On the other hand, Ge—Sb—Te family is constituted by blending two kinds of compounds, i.e. GeTe and Sb2Te3, in various ratio, represented by Ge2Sb2Te5 that is constituted by blending GeTe:Sb2Te3=2:1. Both of the families have been improved through adding other elements etc. and have been applied for broad linear velocities.
For higher speed recording, the delta-phase of Sb—Te is advantageous in light of higher velocity of crystal growth, and lower melting point of the delta-phase of Sb—Te is also advantageous, i.e. its melting point is about 550° C., whereas that of pseudo-binary system of GeTe and Sb2Te3 is no less than 600° C.
In the family where various elements are added into the Sb—Te phase, in general, the crystallization rate may be increased and the recording layer may be applied to higher velocity recording by means of raising the composition ratio of Sb. One disadvantage of the Sb—Te delta-phase is the lower crystallization temperature such as 120° C. to 130° C., which requires that the crystallization temperature should be raised to 160° C. to 180° C. by additional elements such as Ag, In, and Ge to improve the stability of amorphous marks, thereby recording layers are to be produced that are adapted to higher velocity recording up to four times that of DVD.
However, in order to make adaptable to higher velocity recording of 8 times or more than that of DVD, the proportion of Sb should be increased thereby to raise the crystallization rate. In this concept, higher proportion of Sb tends to adversely effect the initialization, for example, reflectance nonuniformity is often induced after the initialization and noise level is exaggerated, as a result proper recordings cannot be performed at lower jitter. Further, the higher proportion of Sb inevitably leads to higher amount of additives since the crystallizing temperature further decreases; merely increasing the amount of additives often results in adverse effects on the initialization, the noise level is possibly exaggerated, as a result proper recordings cannot be performed at lower jitter. As such, in the family based on the Sb—Te delta-phase, it is difficult to produce recording layers that exhibit crystallization rate adapted to higher velocity recording of 8 times or more than that of DVD, provide easy initialization, and satisfy the preservation stability of amorphous marks.
In such backgrounds, Ga—Sb family, Ge—Sb family and the like are proposed that may exhibit higher crystallization rate and superior stability of amorphous marks for the purpose to replace the family based on the Sb—Te delta-phase. Materials of the Ga—Sb and Ge—Sb families show eutectic states at Sb-rich compositions such as above 80 atomic percent of Sb; the materials of Ga—Sb and Ge—Sb families may be employed as materials for higher velocity recording, by utilizing the respective eutectic compositions as the respective main compositions; and these materials may be enhanced the crystallization rate by raising the proportion of Sb, which is similar to the family based on the Sb—Te delta-phase. The crystallizing temperatures of these materials are as high as about 180° C., therefore the stability of the amorphous marks is superior without adding other elements. However, the higher eutectic temperatures such as about 590° C. than that of the family based on the Sb—Te delta-phase possibly result in the insufficient power at recording.
On the other hand, proposals as to optical recording media that employ a phase change recording layer based on In—Sb—Te, or In—Sb-M (M is an element other than In and Sb) may be seen, for example, in Japanese Patent Application Publication (JP-B) No. 3-52651, JP-B No. 4-1933, Japanese Patent (JP-B) No. 2952287, Japanese Patent Application Laid-Open (JP-A) No. 2001-236690, and “K. Daly-Flynn and D. Strand: Jpm. J. Appl. Phys. vol. 42 (2003) pp. 795-799”.
JP-B No. 3-52651 discloses a recording material expressed by the general formula (In1-xSbx)1-yMy (55≦x≦80, 0≦y≦20, M is at least one element selected from the group consisting of Au, Ag, Cu, Pd, Pt, Ti, Al, Si, Ge, Ga, Sn, Te, Se, and Bi); the recording is performed by making use of the reflectance difference between the pi-phase that is a pseudo-stable phase formed by cooling rapidly from a melting condition and a mixed phase or equivalent phase of InSb and Sb formed by cooling slowly from a melting condition. However, formation of the mixed phase or the equivalent phase typically takes a long period. Further, in the proposal, although writing and erasing are allegedly repeatedly capable through scanning laser with variable output, there is no description in terms of the scanning velocity of laser. Accordingly, the formation of equivalent layer is considerably difficult in a condition that the irradiating period at each site is no more than a few decades to a few hundreds nano seconds as that of DVD; therefore the proposal cannot be applied to phase change optical recording media such as CD-RW, DVD+RW, DVD-RW in advanced fashion. Moreover, the proposal is believed not to intend DVD in light of the technical level at the application i.e. 1984, the layer construction or recording way for forming fine amorphous marks of no more than 0.4 μm in length is not described, and needless to say, any disclosure or suggestion cannot be seen with respect to higher velocity recording of 8 times or more than that of DVD.
In the JP-B No. 4-1933, information is recorded and erased through selectively generating two stable conditions by irradiating optical energy with different conditions onto a recording thin film that is formed of fine alloy crystal containing 20 to 60 atomic percent of In and 40 to 80 atomic percent of Sb. One or more elements selected from Ag, In, Ge, Te and the like may be additionally included to the recording thin film in an amount of no more than 20 atomic percent. In the recording thin film of JP-B No. 4-1933, both of the stable conditions are of crystal state having different optical properties, which is allegedly derived from different depositions of In50Sb50 and Sb through the different heating and cooling steps; and the other factors are exemplified such as different size of crystal grain, shape alternation of thin film, generation of different crystal phase and the like. Provided that such factors cause an optical difference and the reflectance may be varied, the difference level of the reflectance is lower and C/N is remarkably lower than those based on phase change between crystal and amorphous being employed in the phase change recording media such as CD-RW, DVD+RW, and DVD-RW, consequently the materials are considered not to be practical.
In JP-B No. 2952287, recording is performed by means of recording material comprising 33 to 44 atomic percent of In or Ga, 51 to 62 atomic percent of Sb, and 2 to 9 atomic percent of Te by making use of phase changes between amorphous and crystal states; allegedly the signal intensity and amorphous stability are sufficient, and erasing can be performed at higher velocity. However, the intended linear velocity of recording is about 1 to 15 m/sec, that is, the crystallization rate is insufficient at higher velocity recording of 8 times (about 28 m/sec) or more than that of DVD, causing a problem that the incompletely erased amorphous marks remain.
The article “K. Daly-Flynn and D. Strand: Jpm. J. Appl. Phys. vol. 42 (2003) pp. 795-799” discloses recording materials expressed by the general formula Inx(Sb72Te28)100-x (in which, x is 3.9 to 45 atomic percent) and recording by means of the material by making use of the phase changes between amorphous and crystal states. However, since the linear velocity of recording investigated in the article is about 2 m/sec to 6 m/sec, the crystallization rate is insufficient at higher velocity recording of 8 times (about 28 m/sec) or more than that of DVD, resulting in a problem that the incompletely erased amorphous marks remain.
In JP-A No. 2001-236690, CD-E media are proposed wherein alloy expressed by Mw(SbxTe1-z)1-w (in which, 0≦w<0.3, 0.5<z<0.9, M is at least one element selected from the group consisting of In, Ga, Zn, Ge, Sn, Si, Cu, Au, Ag, Pd, Pt, Cr, Co, O, S and Se) are employed at the recording layer of a thin film. However, this proposal is not intended the application of DVD, and any disclosure or suggestion cannot be seen with respect to higher velocity recording of 8 times or more than that of DVD (about 28 m/sec or more).
In the In—Sb family, higher proportion of Sb tends to bring about higher crystallization rate similarly to Sb—Te delta-phase, Ga—Sb, and Ge—Sb families. Accordingly, the Sb proportion higher than the compositions disclosed in JP-B No. 2952287 and “K. Daly-Flynn and D. Strand: Jpm. J. Appl. Phys. vol. 42 (2003) pp. 795-799” may lead to the crystallization rate sufficiently adaptable to higher velocity recording of 8 times or more than that of DVD. Also, the melting point of Sb-rich material is about 490° C. (measured by means of DSC the thin film formed by a sputtering process), which is similar to the eutectic temperature even when the Sb proportion is higher than that of eutectic composition. Also, the crystallizing temperature is as high as 180° C. to 200° C., the stability of amorphous marks are superior even without adding other elements.
However, the Sb proportion higher than the compositions disclosed in JP-B No. 2952287 and “K. Daly-Flynn and D. Strand: Jpm. J. Appl. Phys. vol. 42 (2003) pp. 795-799” in In—Sb family leads to the disadvantage that the crystal stability is poor even though the amorphous stability is proper. FIGS. 7A and 7B explain the reflectance decrease at an un-recorded portion or crystal portion of In35Sb65, which is one of the In—Sb family and the composition is near the eutectic composition, after preservation test at 80° C. for 100 hours. FIG. 7A shows that of before the preservation test, and FIG. 7B shows that of after the preservation test. The results show the reflectance decrease of 18% to 29%, i.e. as much as 10% or more, indicating problems that the reflectance does not possibly satisfy the specs, the recording will be inferior since the recording under the condition with lowered reflectance leads to remarkably deteriorated jitter.
Further, JP-B No. 2952287 describes that “the recording element turns into unstable when the content of Q (In or Ga) is 34 atomic percent or less.” The description is considered to mean the instability of crystal phase, and the family having lower In content than the content is considered not to be available for recording layers. However, it is found that the lower content of In or higher content of Sb makes possible to reduce the reflectance decrease after the preservation. Moreover, even when the reflectance decrease is relatively a little, the jitter may grow in the evaluation that the shelf property after preservation test is measured through recording in the same conditions with that of immediately after the initialization. Also, the material, of which reflectance decrease is reduced by increasing Sb content, causes some alternation in the crystal state at least in some degrees, there exists a problem that proper recording cannot be performed under the same conditions with those of immediately after the initialization.
Accordingly, the optical recording media and the related technologies adapted to higher velocity recording of 8 times or more than that of DVD (about 28 m/sec or more) based on recording layer materials that have lower melting point, higher crystallization velocity, and less reflectance nonuniformity after initialization, and exhibit superior crystal stability are not sufficiently satisfactory yet, therefore the provisions are demanded currently as advanced needs.